1. Field of the Invention
This invention relates to electrical capacitors and has particular reference to low cost, relatively small variable or adjustable capacitors.
2. Description of the Prior Art
As is well known, the capacitance value of a capacitor is proportional to the area of the conductor plates and inversely proportional to the thickness of the dielectric material therein.
Heretofore, fixed capacitors of relatively high capacitance value have been formed of a thin plastic or paper dielectric material on which a thin layer of a conductive material is bonded. Two or more such films are interleaved and wrapped in a coil to form the capacitor. In view of this form of construction, such capacitors can be made into small or miniature sizes.
On the other hand, variable capacitors have generally been constructed of conductor plates or elements which are movable relative to each other and separated to receive a suitable dielectric fluid, such as air or other gaseous or liquid substance, there-between. Since the conductor elements must be separated, they must be made rigid and accurately and precisely spaced. Also, air and other fluid dielectrics have relatively low breakdown voltage characteristics, requiring a relatively large spacing between the conductor elements. Thus, such variable capacitors are comparatively bulky, heavy and expensive to manufacture and cannot generally be made in very small or miniature sizes. Furthermore, in many cases, elaborate sealing means must be provided to prevent contamination or dilution of the gaseous or liquid dielectric by the exterior environment which could cause eventual malfunction or breakdown of the capacitor.
Various attempts have been made to overcome the above-noted disadvantages of prior variable capacitors. For example, U.S. Pat. No. 3,284,682 issued to H. E. Lippman on Nov. 8, 1966, discloses a variable capacitor in which rigid plates are spirally formed and interleaved with each other, with defined spacing therebetween to receive a dielectric gas such as air or the like. The plates are adjusted axially relative to each other to vary the capacitance. In this case, the plates and other operating parts must be made and assembled with a high degree of precision and accuracy and the conductor plates must be precisely spaced to maintain consistent results and to prevent breakdown of the dielectric which might otherwise occur if the distance between the plates is allowed to vary. Thus, the resulting capacitor tends to become relatively large and expensive to manufacture and is subject to malfunction if wear should occur in the operating parts.